Hypodermic needles are used in medicine both to inject liquid materials into the body and to withdraw samples from the body. The usual withdrawal is the withdrawal of venous blood. Many infectious diseases are carried in the blood. It has become important in the medical arts to protect the practitioners from contact with possibly-infectious blood. Skin surface contact with blood and other body fluids is not particularly harmful, especially when the skin is healthy. The larger danger for medical practitioners is the possibility of being inadvertently stuck by a needle which carries on it elements of another person's blood. Quite a number of different apparatuses and protocols have been created to minimize risk of inadvertent sticking by a used hypodermic needle. These dangers require the need for advances in the protection from hypodermic needles, and particularly an operative system whereby the hypodermic needle is automatically withdrawn into the retracted position when the needle assembly is unmounted.